Here's what some of the Panorama from CS:GO looks like. There's also some documentation on Valve's wiki. This isn't what it looks like in Sandbox.
Layout File
The layout file is an xml file. It kind of looks like html but it's harder coded. It can pull in styles and javascript files.
attribute 'contenteditable' not allowedattribute 'blocktype' not allowedattribute 'title' not allowedattribute 'data-image-style' not allowedStylesheet File
The stylesheet files are an enhanced css, they have new/different properties and @defines.
attribute 'contenteditable' not allowedattribute 'blocktype' not allowedattribute 'data-image-style' not allowedattribute 'title' not allowedJavascript File
This is standard javascript with some exposed game engine bindings.
attribute 'contenteditable' not allowedattribute 'blocktype' not allowedattribute 'data-image-style' not allowedattribute 'title' not allowed